Biological Significance of Treponema pallidum Surface Antigens. The goal of this work is to define and purify individual T. pallidum molecules of relevance to the pathogenesis of syphilis. We have expressed in E. coli a 190 kilodalton T. pallidum antigen which is remarkably resistant to proteolytic degradation. Twenty-five milligram amounts of this antigen, designated "4D" have been purified to homogeneity. Immunization of rabbits with the 4D antigen induces a complement dependent serum activity which immobilizes T. pallidum in vitro. We will present plans to develop an optimal immunization protocol for maintenance of high levels of immobilizing activity and then test the relationship between immobilizing activity (TPI), immunity to dermal challenge with virulent T. pallidum, and the ability of the organism to attach to host surfaces. We plan use of immunoelectronmicroscopy (IEM) to localilze the 4D antigen and other cloned antigens after thin sectioning of T. pallidum. Cloned T. pallidum antigens with IEM demonstrated surface location will be studied for activity in systems to assess attachment and immunity in experimental syphilis.